Glass display panels are used in a variety of applications—from hand-held mobile devices to tablets to computer monitors to television displays. These applications require glass sheets which have pristine, defect-free surfaces.
One method of producing glass for optical displays is by an overflow downdraw process (also known as a fusion downdraw process). This process produces pristine surface quality compared to other processes referred to as the float and slot techniques in the literature. U.S. Pat. Nos. 3,338,696 and 3,682,609 (Dockerty), which are incorporated in their entirety herein by reference, disclose a fusion downdraw process which includes flowing a molten glass over the edges, or weirs, of a forming wedge, commonly referred to as an isopipe. See also U.S. Patent Publications Nos. 2005/0268657 and 2005/0268658, the contents of which are also incorporated herein in their entireties by reference. The molten glass flows over converging forming surfaces of the isopipe, and the separate flows reunite at the apex, or root, where the two converging forming surfaces meet, to form a glass ribbon, or sheet. Thus, the glass which has been in contact with the forming surfaces is located in the inner portion of the glass sheet, and the exterior surfaces of the glass sheet are contact-free.
The sheet as it evolves decreases in thickness under the forces of gravity and pulling equipment. In particular, pulling rolls are placed downstream of the isopipe root and to adjust the rate at which the ribbon leaves the isopipe, and thus help determine the thickness of the finished sheet. The pulling equipment is located sufficiently downstream so that the viscous glass has cooled and become rigid enough to be pulled. The contacted edge portions are later removed from the finished glass sheet. As the glass ribbon descends from the root of the isopipe, it cools to form a solid, elastic glass ribbon, which may then be cut to form smaller sheets of glass.
The glass sheet that is produced by the fusion downdraw process, however, has a narrower width than the viscous glass ribbon at the root. This loss of width is due to contraction of the glass ribbon laterally within the viscous region of the drawing process, which is also referred to as sheet width attenuation. Contraction of the ribbon laterally during the viscous phase of the drawing process is also related to an instability known as sheet width variation, which may be characterized by unstable velocity contours within the viscous glass sheet.
The loss of sheet width within the viscous region of the downdraw process also manifests itself as accumulated thickness, or beads, at the edges of the sheet. Due to the differences in both thickness and temperature between the beads and the center of the sheet, the formation of these edge beads gives rise to a multitude of issues. For example, the formation of edge beads can cause temporary stresses that produce an unstable ribbon shape during the draw process and/or permanent stresses in certain regions of the sheet upon cooling of the glass. The edge beads may also prevent an operator from bending the glass ribbon to a desired radius of curvature, such as may be necessary where the glass is prepared for use in certain applications. Thus, there is a need in the industry to address these problems.